A polycrystalline zinc oxide member whose constituent elements are zinc and oxygen is expected to apply in a photo-detector, a surface acoustic wave element, a piezoelectric element, a transparent conductive electrode and an active element. As a method of fabricating the same, many methods such as an MBE method in an ultra-high vacuum, a laser aberration method, a sputtering method, a vacuum deposition method, a sol-gel method, and an MO-CVD method are under study.
As to an active element such as a transparent transistor and so on, from a point of view of the crystallinity, a method in which the MBE method in an ultra-high vacuum and the laser aberration method are combined is a present main stream. However, the method is not a preferable method in realizing a low cost and large area active element. In contrast, in the fabrication of a transparent conductive electrode, the method is usually used and top data are obtained therewith. Furthermore, in large area deposition, the sputtering method is tried. However, low resistivity comparable to that due to the MBE method has not been obtained. From these present situations, an MO-CVD method (Metal Organic Chemical Vapor Deposition System) that is capable of forming a large area highly crystalline thin film is attracting attention.
Minami et al. disclose, in Appl. Phys. Lett., 41 (1982) 958, the preparation of a zinc oxide material from an organometallic compound of a beta-diketone compound by use of the MO-CVD method. The disclosure thereof is confined to using the material as a transparent conductive electrode and relates to a polycrystalline zinc oxide thin film that is formed on a glass substrate and has c-axis orientation. In addition, the resistivity thereof is also insufficient. Furthermore, there is no report of applications of the fabricating method to other elements.
An example in which with the MBE method zinc oxide is applied to a transparent transistor is reported by Kawasaki et al. of Tokyo Institute of Technology in Proceedings of 2000 JSAP (the Japan Society of Applied Physics) Spring Meeting. In the report, though as the substrate a glass substrate is used, crystalline orientation is a c-axis of a wurtzite structure.
Unlike a solar battery in which conduction in a depth direction of a thin film is used, the transparent transistor, transparent conductive electrode and surface acoustic wave element make use of conduction in an in-plane direction. Nakamura et al. of Tohoku University report that when the MBE method is used, a single crystal zinc oxide thin film having a-axis orientation is suitable for the surface acoustic wave element (Jpn. J. Appl. Phys 39 (2000) L534).
A zinc oxide (ZnO) thin film formed on a polycrystalline or amorphous substrate, in almost all cases, exhibits c-axis orientation of a crystal axis. It is considered that this is due to a strong ionic bond of Zn and O. As a method of obtaining a zinc oxide thin film having a-axis orientation, the present inventors proposed a DC-bias sputtering method and reported in Proceedings of 45th Meeting of JSAP, 30aPB/II, p. 630 (March/1998). According to the method, with a substrate made of a conductive metal or an insulating substrate such as glass on a surface of which a conductive thin film such as ITO or Al is formed, a DC bias is applied on the conductive thin film, and thereby a thin film of zinc oxide is formed. According to the method, orientation control to an a-axis is made possible. However, the crystallinity is not sufficient. Furthermore, when a transparent thin film transistor is formed with the substrate of the a-axis orientation, a configuration as shown in FIG. 1 is obtained. In the transparent thin film transistor, on a zinc oxide thin film 30 having the a-axis orientation, a source electrode 42, a gate oxide film 43, a gate electrode 44 and a drain electrode 45 are formed. However, in the transparent thin film transistor, since a conductive thin film 20 for applying a DC bias is formed on the glass substrate 10, when a current is flowed between the source electrode 42 and the drain electrode 45, the current that should originally flow within the zinc oxide thin film 30 flows within the conductive thin film 20 such as ITO or Al that is very low in the resistance in comparison with zinc oxide. As a result, there occurs a structural problem that the transparent transistor does not at all work.
In the next place, as means of improving the crystallinity, the present inventors tried to epitaxially grow a polycrystalline zinc oxide thin film, with a zinc oxide thin film that is formed according to the above method and has a-axis orientation as a seed crystal, on an upper portion thereof by use of an atmospheric MO-CVD method, and obtained high crystal orientation. The above results are reported in Proceedings of Tohoku Regional Meeting of the Surface Science Society of Japan G1 P32 (November, 1998). However, it does not mean that the above structural problem caused in applying to the transparent transistor can be improved.
One of applications of the zinc oxide thin film having a-axis orientation is an electrophotography photoreceptor. The zinc oxide thin film having the a-axis orientation has a structure in which columnar crystals fall down. When a zinc oxide thin film is formed thereon by use of the MO-CVD method, larger grains grow. This allows improving photosensitivity of the electrophotography photoreceptor. There is a report by D. H. Zhang, D. E Brodie et al. in which they verified an improvement of the photosensitivity (Photoresponse of polycrystalline ZnO films deposited by r, f, bias sputtering; Thin Solid Films 261 (1995)334-339). The report explains that photosensitivity of a-axis orientation is remarkably higher than that of c-axis orientation. The electrophotography photoreceptor, in its sectional view, has a structure as shown in FIG. 2. As the operation mechanism thereof, firstly, a surface of a zinc oxide thin film 70 is provided with electric charges, by an electric field generated by the electric charges, light carriers that are pair-generated by an external incident light are separated in a direction vertical to a substrate 60. In FIG, 2, since positive electric charges are charged on a surface, electrons that are ones of pairs of light carriers generated by light are attracted to the surface and positive holes that are the others thereof are attracted toward the substrate. In order to generate such an electric field that is vertical to the substrate, there has to be a conductive substrate for supplying a potential to a surface of the substrate 60. So far, a drum shaped Al plate that is a conductor of low resistance is used. In place of the Al plate, ITO or Al that is a conductive thin film may be formed on an insulating substrate. From the above, it goes without saying that forming a conductive thin film on a glass substrate in order to form a zinc oxide thin film of a-axis orientation, though being a grave disadvantage in the application to the transparent transistor, is indispensable for the electrophotography photoreceptor. By summarizing examples of applications to the electrophotography photoreceptor, the present inventors applied a patent under a name of “Photoreceptor, Electrophotography Apparatus and Photoreceptor Container” in Japanese Unexamined Patent Application Publication No. 2000-137342.
An object of the present invention is to provide a polycrystalline zinc oxide semiconductor member that is high in mobility in a planar direction and has a-axis orientation of a crystal axis, and a method of fabricating the same.
Furthermore, another object of the invention is to provide a zinc oxide semiconductor member that is transparent in the visible region.
In order to accomplish the above objects, in the present invention, a polycrystalline semiconductor member formed on a substrate is characterized in that the member comprises at least zinc and oxygen as constituent elements, and a crystal orientation plane is oriented in a wurtzite structure a-axis.